Field of the Present Invention
The present invention relates to a substrate stage, a lithographic apparatus and a method for manufacturing a device.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. In such stepper or scanner arrangements, an accurate positioning of the substrate relative to the irradiated pattern (provided by a projection system) is required. An accurate positioning is required in both the horizontal plane (in order to assure an accurate match between successive layers of the integrated circuit) and in vertical position (relative to the projection system, in order to assure that the target portion is in focus). In order to accurately position the substrate relative to the projection system a positioning device is provided, enabling to control a position of the substrate in 6 degrees of freedom. In view of an increase in the accuracy requirements for the positioning of the substrate relative to the patterned radiation beam, more complex positioning devices have been designed. Such devices can e.g., include an overdetermined number of actuators in order to take account of e.g., deformations of substrate tables holding the substrate. It is expected that meeting the overlay and focus requirements will become more and more difficult, in particular when a transition is made towards larger wafers or substrates, such as 450 mm wafers. Further, external disturbances as e.g., caused by liquid containing devices that are used in immersion lithographical apparatuses may further hinder an accurate positioning, in particular with respect to focusing of a substrate.